In modern gas turbine engines, working medium gases having temperatures in excess of two thousand degress Fahrenheit (2000.degree. F.) are expanded across rows of turbine blading for extraction of power from the flowing medium. A shroud, termed an outer air seal, circumscribes each row of turbine blading to inhibit the leakage of working medium gases over the blade tips.
Outer air seals of some engines are formed of a metallic substrate to which a thermal barrier coating is applied for protection of the seal from the high temperature, working medium gases. Ceramic materials are generally known to be effective thermal insulators and are in wide use in such seal application. As long as the ceramic coating remains intact, the ceramic prevents unacceptable deterioration of the metallic form to which it is adhered.
Durable structures capable of long term, reliable service in the hostile turbine environment are sought. Specific needs are high temperature capability, and good resistance to thermal shock. Additionally, for turbine seal applications the structure must have adequate surface abradability to prevent destructive interference upon the occurrence of rubbing contact of the seal by circumscribed rotor blades and good erosion resistance, particularly at the leading edge of the seal to prevent excessive wear at the incidence of particles entrained in the working medium upon the seal. In some engines the hot working medium gases alone may be erosive.
U.S. Pat. No. 3,091,548 to Dillon entitled "High Temperature Coatings"; U.S. Pat. No. 3,817,719 to Schilke et al. entitled "High Temperature Abradable Material and Method of Preparing Same"; U.S. Pat. No. 3,879,831 to Rigney et al. entitled "Nickel Base High Temperature Abradable Material"; U.S. Pat. No. 3,911,891 to Dowell entitled "Coating for Metal Surfaces and Methods for Application"; U.S. Pat. No. 3,918,925 to McComas entitled "Abradable Seal"; U.S.Pat. No. 3,975,165 to Elbert et al. entitled "Graded Metal-to-Ceramic Structure for High Temperature Abradable Seal Applications and a Method of Producing Said"; U.S. Pat. No. 4,109,031 to Marscher entitled "Stress Relief of Metal-Ceramic-Gas Turbine Seals"; U.S. Pat. No. 4,163,071 to Weatherly et al. entitled "Method for Forming Hard Wear-Resistant Coatings"; and U.S. Pat. No. 4,289,446 to Wallace entitled "Ceramic Faced Outer Air Seal for Gas Turbine Engines" are representative of the known concepts applicable to ceramic faced seals.
Although many of the materials and methods described in the above patents are known to be highly desirable, the structures resulting therefrom have yet to achieve full potential in hostile environment applications. Of particular remaining concern in outer air seal applications is the balance needed for good abradability in response to blade rubbing contact and good erosion resistance to the effects of particles entrained in the working medium stream.